Hello. I have a question about securing a server ad forwarding some ips

I have a computer connected to the internet using IP eg: 193.233.24.36 and I have a 32 IP subnet - eg 193.233.188.32 with netmask 255.255.255.224 (this means that I can use 193.233.188.32 for braodcast and IPs from 193.233.188.33 -> 193.233.188.63)

The "server" has 2 ethernet cards: eth0 with IP 193.233.24.36 and eth 1 with 193.233.188.33.
I set /proc/sys/net/ipv4/ip_forward to 1 - this ensures the IP forwarding from and to the copmuters in the 193.233.188.32 subnet.
My questions are the following: do I need to add any other line in the firewall configuration, for the computers in the subnet to have access to the internet ?

How can I secure my server from outside attacks and not cut off the internet access for the other computers ?

Please help me with this = I am really new to this thing and I would really like to know a "professional"s idea about this